Monday, September 8, 2014

Imagining Other Earths — The Grand Universe

Growing up as a kid in New York State, I had always enjoyed going into the city and seeing the Hayden Planetarium. That's why I was really pleased when I was asked to be part of a group of astronomers, who were asked to help them redesign and think about how to create the new stunning planetarium that's in New York today. When we were gathered in this committee, we were asked by the designer to summarize all of astronomy in a single line.

At first, this seemed incredibly daunting: to take all of the science we've learned from the ancient Babylonians, the Egyptians, Galileo, Newton, Einstein, the most recent results from the Hubble Space Telescope, and reduce it all to a single sentence.

We sat there for three hours, struggling to summarize all the things that we've done, I think, as any scientists could talk for hours about the importance of their particular contribution. We sat down and we finally came up with a single sentence, the theme of today's lecture, which for me is the most important thing that people should know about astronomy. And that's the idea that the universe is big.

And this is the first of a series of lectures in our class. The class will consist of really three main parts. We'll begin by exploring our own solar system. At the end of that section, we'll have an assignment where you'll write a paper or make a presentation about one of the ideas for building new satellites to explore our solar system.

The second section will be focused on stars and the new planets that we're discovering around these stars. That will end with a assignment where you'll write a paper or develop a presentation on some of the new missions we're hoping to do to discover planets around nearby stars.

And then the final section, we'll return to the Earth and look at the history of life on the Earth, and how it evolved, how evolution works, and then apply those ideas to extrasolar planets, and ponder how life might evolve elsewhere.

The final assignment, I think, will be the most exciting and the most challenging as well. You will create your own solar system. Imagine how life evolves on that solar system. Take some of the lessons we've learned from looking at Mars and Europa and Earth, and apply it to your new planetary system.

I want to begin this course with two introductory lectures. Today's lecture will talk about the universe, and tomorrow's lecture, when we'll talk about the origin of life, what is life, and how we might detect it. Today's lecture, and many of the lectures will consist of several parts, you'll have a segment of about 15 to 20 minutes. That would be followed by one or two short questions, you'll answer those questions then return to the lecture. There'll be another segment of about 15 to 20 minutes, another set of questions, and then the final part of the lecture.

In this lecture and in some others, I'll try to embed some videos that I'd like you to look at, either during the lecture or at the end of the lecture. So let me now begin here and talk about our place in the universe.

We’ll begin first with our own solar system, then turn to our galaxy, and finally look at our place in the universe as a whole. So we're going to start with the Earth. Here we are, on Earth, we're right here in New Jersey. You're spread out somewhere else on this planet. Let's just begin by looking up some basic facts about the Earth.

What I'd like you to do is actually pause the lecture for a moment, go into Google and type in the following things: Earth mass, Earth radius, and how far is the Earth from the Sun? One of the wonderful things about having the web today is you can look up all of the basic astronomical numbers that you might need for any of the problems that we'll look at in this class. So later on, we'll be looking at things like how far is Mars from the Sun. If you want to know that right now, type that question into Google, what will come up is that distance.

Also, all the basic constants of nature, the speed of light, the strength of gravity, all those numbers will come up immediately if you type in gravitational constant or light speed on the web.

This is extremely convenient. In the old days, you would have a textbook for your introductory astronomy class that had a list of all those numbers. Now, they're immediately available to all of you. And I will not be providing them in the questions I'm asking. If you want those numbers, you should look them up. I hope you have.

Planet: Earth
Mass: 5.972E24 kg
Radius: 3,959 miles (6,371 km)
Distance from the Sun: 92,960,000 miles (149,600,000 km)

I want to start with this number, how far is the Sun from the Earth? That distance is what we call an astronomical unit. It's about 150 million kilometers. And, of course, 2 pi times that distance is roughly the distance the Earth travels as it orbits around the Sun. That will be our measure, our ruler for looking at the distance to stars, and for when we talk about other solar systems and we compare how far their planets are from the star.

We will often want to talk about the distance that a planet is from its star in astronomical units. And if a planet is one astronomical unit away from its star, its distance is the same as the Earth from our Sun

Some of the planets we discover by direct imaging, we'll find are 10 or 20 astronomical units away from their star, so they're much further out, they'll be colder planets, much like Saturn or Uranus. The Kepler Telescope has been exploring the inner regions of solar systems, and it has discovered many planets at half an astronomical unit, a third of the astronomical unit, even a tenth of an astronomical unit, much closer to the star than Earth.

Now let's turn to planets. I hope that tonight, you all go out and look for planets. And let me teach you the first thing you need to know to be an expert astronomer: stars twinkle, and planets don't. So when you look at the night sky, if you see something that's bright and flashing and moving, that's an airplane. If you see something that's twinkling, it's a star. The brightest twinkling star you'll see in this time of year, is probably Sirius (not the Prisoner of Azkaban) which is the brightest star in the sky.

And depending on when you look at night, you'll see different planets that are up. Between Venus and Mercury, Mercury is hard to see but Venus is quite bright. Venus is always close to the Sun, so depending on the time of year, you will either see Venus soon after sunset or right before sunrise. Jupiter is the other very bright star, well, bright planet. And, it's often seen directly overhead.

If you want to see what's up in tonight's sky, let me just recommend two websites, and I would encourage you to go look at these websites tonight. Go out and if it's a dark, clear night, see what you can see, see if you can find some planets yourself.

If you have binoculars, I encourage you to look at Jupiter, and if you're lucky, you'll see not just Jupiter, but its massive moons. These moons were first discovered by Galileo when he turned his telescope to the night sky.

When the ancients stared up at the night sky, they noticed that some stars stay fixed, and some, what they thought were stars, moved. They call these ones that moved, wanderers, and those wanderers were planets. And planets, their positions across the sky change, and this change and this motion takes place because while the stars are far away, the planets are close. So the relative positions of the Earth and the other planets move as they orbit around the Sun. And this is why Mars or Jupiter or Saturn appear at different positions in the sky as they move around their orbits throughout the year.

So let me encourage you, and these are questions that I want you to go off and do after lecture, because it requires going, getting away from your computer, stepping outside, and looking at the night sky, see if you find any planets and go stare at the Moon. See what the phase of the Moon is, and watch, over the next month, how it changes.

If you have access to binoculars or small telescope, point it at the Moon. See its craters? It's this remarkable fact that so excited Galileo, that the nearby planets were not perfect. The moon wasn't the perfect surface, but it's this crater structure, those craters, that reflect the whole history of the formation and evolution of the solar system.

So, here's our solar system with the Sun in the center, followed by Mercury, Venus, Earth, and Mars. These are what we call the inner planets. And you'll notice they fill this tiny region. The distance from the Sun to the Earth, our astronomical unit, is much smaller than the distance from the Sun to Neptune. So as we move out, we then have Jupiter, Saturn, Uranus, and Neptune. And the inner solar system is a much smaller region than the outer solar system. This is actually an old figure, and it includes Pluto as one of the planets of the solar system. As I'll argue later, Pluto really ought to be grouped in with the other dwarf planets. Here again is the outer solar system, Jupiter, Saturn, Uranus, and Neptune. And you can see Pluto is different from the others. It's on a very different kind of orbit, much like the other dwarf planets that have been discovered in the past decade. Here's one of them Eris, whose properties are much like Pluto. And Eris is but one of the many objects, in what we call this Kuiper belt made, built off of remnants the formation of the older, early solar system.

This is another way of displaying a map of the solar system. But here we're using what we call a logarithmic scale. So as we move out here, we're looking at factors of ten. The distance from the Sun to the Earth is one astronomical unit, Saturn is ten astronomical units, so on and so forth. And we're using scientific notation here where 10 is written as 10 to the 1 power. The distance out to what we call the terminator, the boundary between the material that we find in the solar system and the interstellar material is about at 100 au.

This year, a remarkable event took place in our exploration of the universe. The Voyager spacecraft crossed the terminator, crossed the boundary between our solar system and the material outside. Now, while it's crossed this point, it still has a very long way to go to get to the nearest star. The nearest star's not 1,000 au, or 10 to the 4 au, the nearest star, Alpha Centauri, is out here at more than 100,000 astronomical units from the Sun.

So you can see the distance to the nearest star is much, much further than the distance to the planets, right? That's a factor of about 10,000. And Alpha Centauri is the nearest star, the typical star we can see in the night sky is often a million, or often 10 million astronomical units away.

Now I'd like to turn to the first problem, and the goal of these problems are twofold. One, it's a chance to improve your quantitative reasoning skills. Two, it's important to sit down and think about some of the concepts that we're talking about in the class, make some rough estimates, and get a feel for some of the numbers we're talking about.

In astronomy, we deal with really big numbers. And I think one of the ways to make sense of them is to start thinking them through. The first question we'll do is we'll look at how long it takes to travel from the Earth to Mars, at their distance of closest approach in a rocket. And to make this problem easier, we're going to keep Mars and Earth fixed. So the way to estimate this is to go look up the Earth's distance from the Sun, look up Mars's distance from the Sun, take the difference between the two, divide by our velocity and figure it out.

You should have found on that problem that it'll take roughly 100 days to travel from Earth to Mars if neither planet moved. The real situation is a bit more complicated: the Earth moves on its orbit around the Sun. Mars moves on its orbit around the Sun. So if we are to travel from Earth to Mars, we'll have to go on one of these transfer orbits. Because the path is somewhat large, longer than moving on a straight line, it will take roughly 200 days to travel from Earth to Mars. This is much longer than the roughly four days that it takes to go, say to the Moon. And because of this longer travel time, this mission is about at the edge of what we can do technologically for the moment today. Here's to hoping for a better tomorrow!

That said, like many people interested in space exploration, I think that Mars is the next obvious destination for human exploration. As we'll see later in the course, there's a tremendous amount that we'd like to learn about Mars. We've gained a great deal of knowledge from our rovers and orbiters that have gone to Mars, but it'd be very exciting to be able to send humans.

This 200-day journey is going to be very challenging. We're going to have to worry about a hostile space environment. And this will be a great challenge, but one that I hope we rise to within our lifetimes.

— Professor David Spergel, Princeton University

No comments:

Post a Comment